Air conditioning blows but sometimes cools and sometimes does not

The air conditioning always blows just fine. It will stop cooling on occasion. I have had it into a repair shop several times and they can never find a leak. It cooled fine this last time for six months before it stopped cooling once again. The repair shop is convinced there is a leak but when they pressure the system a leak does not show up. Sometimes the air conditioning will cool for a couple of days and then stop. You never know when it will give out or if it will be for a long time or for just a few hours or a couple of days.